1 Introduction

The high conductivity and light weight of conductive papers or films are important features for the development of electronic devices. A general strategy for the production of conductive papers or films is to build conductive networks using conductive materials in or on cellulose paper or a flexible matrix (usually macromolecular network-based elastomers)[1-2].Nevertheless, continuous conductive networks are rather difficult to form because of high steric hindrance and percolation ability, which severely limits their conductivity performance in the resultant papers/films. For example, Choi H Y et al used a layer-bylayer strategy to dope silver nanowires and carbon nanotubes (CNTs) onto cellulose paper[3]. As the conductive network was built on the surface of the paper, the conductivity of the paper reached as high as 1 S/cm; however, the conductivity of the papers made using this strategy was easily affected by scratches on the paper surface that exposed the conductive layer.Chang-Jian C-W and coworkers used an aerogel of preformed multi-walled carbon nanotubes (MWCNTs)to improve the dispersion of the MWCNTs in either a polydimethylsiloxane (PDMS) matrix or a poly(3,4-ethylene-dioxythiophene): poly(styrene sulfonate)(PEDOT:PSS) matrix and prepared a conductive paper with low MWCNT content[4-5]. The results showed that as little as 5 wt% or 10 wt% of MWCNTs could be dispersed in the matrix to form conductive papers with conductivities of about 100 S/cm to 1000 S/cm,respectively. Furthermore, Gan and coworkers studied the percolation behavior of CNTs in a PDMS matrix in detail and successfully predicted the percolation threshold of CNTs in the PDMS matrix[6]. It was found that a high mass ratio of conductive fiber was required to make a conductive paper with high conductivity.

Graphene oxide (GO) and reduced graphene oxide(RGO) are layered carbon nanomaterials that have high surface areas and are flexible and conductive[7-11].Their hydrophobic surfaces usually allow GO and RGO to disperse easily in organic solvents and be used to build in situ conductive networks in conductive papers.Compared to CNTs, which are typically seriously self-entangled and hard to disperse in solvents, GO can be easily dispersed in a uniform manner to form continuous networks[2,12]. Kang et al used graphene and CNTs to build a transparent electrode from films with a resistance of ～2.4 kΩ/cm2[13]. Bai et al also used pure GO to make a transparent conductive paper having a conductivity of 3.7×10-5 S/cm[14]. The conductivity of the material could be further improved by reducing the GO (or RGO) content, which could then be better applied in the preparation of conductive films[15].However, the load ratio of GO in these materials was still high relative to its large size; this needs to be addressed to make GO-containing papers conductive.

Tunicate-derived cellulose nanocrystals (TCNCs)have a high aspect ratio and exhibit high mechanical strength[16-17]. Their hydrophilic surfaces make the formation of hydrogen bonds among TCNCs easy,which suggests that TCNCs could be used to obtain conductive papers with high mechanical strength. Wen et al used CNCs and RGO to prepare a composite conductive paper with ultra-low resistance (1.48 Ω/cm2)[18],in which the CNCs greatly enhanced the mechanical strength of the paper. The toughness of this paper was as high as 11 MJ/m3. Zhao et al used glucomannans with TCNCs to develop a tough biomass film[19]. The Young’s modulus of the paper and tensile strength of the film were found to be 11.71 GPa and 55.56 MPa,respectively. However, to the best of our knowledge,TCNCs have not been used before for the preparation of conductive papers.

Cellulose is a green material that has been previously employed to produce conductive papers[20-22].Herein, inspired by the abovementioned results, we dispersed polydopamine-coated GO into an oxidized TCNC suspension, cast the mixture, evaporated the solvent, and prepared a conductive paper with good mechanical properties and a low GO content. In our strategy, GO was first coated with polydopamine (PDA)to improve its surface hydrophilicity. The electrostatic interaction of the amino groups on PDA molecules and carboxyl groups on the oxidized TCNC (OTCNC)surface further improved the stability of the GO sheets in the TCNC suspension. TCNCs were oxidized by 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) and the PDA-coated GO aqueous suspension was subsequently mixed with the OTCNC aqueous suspension, followed by casting and evaporation to make conductive papers.The advantage of this strategy is that the hydrogenbond cross-linked particle-based network of OTCNC papers is better for the formation of a continuous GO conductive network, while the high mechanical strength of OTCNC provides the paper with good mechanical properties.

2 Experimental

2.1 Materials and methods

GO was obtained from Changchun Institute of Applied Chemistry, Chinese Academy of Science. Dopamine hydrochloride and 1,1,1-tris(hydroxymethyl)-methylamine (Tris, 99%) were purchased from Aladdin Chemical Co. Tunicates were collected at Weihai,Shandong province, China. H2SO4, KOH, NaClO, acetic acid, EtOH and NaBr were procured from Sinopharm Group Co. Ltd. and 2,2,6,6-tetramethylpiperidinyloxy(TEMPO) was purchased from Sigma-Aldrich Co.

The morphologies of TCNC, GO, and their derivatives were characterized using an H-700FA(Hitachi) transmission electron microscope (TEM).The results of surface modification of TCNC and GO were characterized using a Nicolet 6700 (Nicolet Instruments) Fourier transform infrared spectrometer(FT-IR, wave number 400～4000 cm-1). The crystal structures of TCNC, GO, and their derivatives were characterized using a D8 Advance (Bruker) X-ray diffraction spectrometer (XRD) and the constituent elements were characterized using ESCALAB 250Xi(Thermo Fisher Scientific) X-ray photoelectron spectroscopy (XPS). The mechanical properties of the conductive papers were characterized on a CMT6503(SANS Co.) universal testing machine, and the conductivity was measured with a Keithley 6517B(Tektronix Inc.) high resistance meter.

2.2 Extraction of TCNC

To extract TCNCs, tunicates (10 g) were dried (60℃,3 days), ground, and treated with 5 wt% KOH aqueous solution (300 mL) for 12 h, and then dried at 60℃ for 3 days. The KOH-treated tunicate was then bleached twice using acetic acid (10 mL) and NaClO (7.1 mL),and subsequently dried. The bleached tunicate was hydrolyzed using 40% H2SO4 at 40℃ for 4 h, then washed with distilled water via centrifugation, and dialyzed to yield TCNCs. The product was centrifuged and freeze dried to obtain purified TCNCs (white powder, yield of 50%).

2.3 Synthesis of oxidized TCNCs

TEM and XPS were used to study the effects of these modifications on the morphology of the nanomaterials (Fig.2). The TCNCs were 1～5 μm long with a diameter of 10～20 nm (Fig.2a). After oxidation,the morphology of the TCNCs was not changed(Fig.2b). The XRD spectra of TCNC and OTCNC also showed the same trend (Fig.2c), with the XRD spectrum of OTCNC overlapping well with that of TCNC. The peaks located at 2θ=14.7°, 16.5°, 22.7°,and 34.4° indicated that both TCNC and OTCNC were in a good β-crystal format. The crystallinities of TCNC and OTCNC were 87% and 81%, respectively,which also indicated that oxidation did not damage the crystal structure of TCNC. Coating GO with PDA obviously increased the thickness of the GO sheet,which was observed by TEM (Fig.2d and Fig.2e). The XRD spectra of GO and PDA-GO showed the same trend. The peak in the GO XRD spectrum located at 2θ=10.7° shifted to 8.9°, which indicated that the distance between the GO sheets had increased from 0.84 nm to 0.99 nm. Meanwhile, a new peak was found at 26.1° after the coating of GO, which indicated that GO was partially reduced.

2.4 Synthesis of polydopamine-coated GO

To make polydopamine-coated GO (PDA-GO), GO(100 mg) and dopamine hydrochloride (50 mg) were added into 10 mmol/L Tris-HCl buffer (200 mL, pH value of 8.5) and ultrasonicated in an ice bath for 10 min to allow the GO to disperse uniformly. The mixture was then stirred at 60℃ for 24 h and filtered until the filtrate was clear and colorless. The filtrate was freeze dried to yield PDA-GO.

The results of the oxidation of TCNCs and coating of GO were easily determined by FT-IR spectra(Fig.1). After oxidation, a band appeared at 1724 cm-1 and was attributed to the C=O of carboxyl groups.This band was not present in the FT-IR spectrum of TCNCs. TEMPO oxidized the C6—OH group into carboxyl, which then formed hydrogen bonds with other hydrophilic groups more easily than the hydroxyl groups. Meanwhile, the existence of carboxyl groups also made the TCNC surface more negative, which improved its stability in water. On coating GO with PDA, two new bands at 1568 cm-1 and 1220 cm-1 and attributed to the N—H and phenyl ring, respectively,were observed in the FT-IR spectrum of PDA-GO and indicated that PDA had successfully coated the GO surface.

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2.5 Preparation of conductive paper

Acknowledgments

3 Results and discussion

3.1 Synthesis of OTCNC and PDA-GO

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Fig.1 FT-IR spectra of TCNC, OTCNC, GO, and PDA-GO

Fig.2 TEM photographs of (a) TCNC, (b) OTCNC, (d) GO, and (e) PDA-GO; (c) and (f) XRD spectra

The oxidized TCNCs were prepared following the literature procedure[23]. TCNCs (2 g) were dispersed in 200 mL of distilled water to make a 1 wt% TCNC suspension. NaBr (652 mg) and TEMPO (59 mg) were then dissolved in 200 mL of distilled water and added to the TCNC suspension. Subsequently, NaClO (15 g,Cl=14.5%) was added to the mixture and the pH value was adjusted to 12 using 0.1 mol/L NaOH aqueous solution. After stirring for 4 h, EtOH (4 mL) was added to the mixture to terminate the reaction. The product was washed thrice with distilled water and then added into 1 mol/L HCl and stirred for 30 min. This was followed by washing with 0.5 mol/L HCl and distilled water until the pH value of the solution decreased to 7. The solution was then freeze-dried to yield pure oxidized TCNC (OTCNC).

3.2 Mechanical properties of the conductive paper

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Table 1 Tensile strength and elongation at break of conductive papers containing different PDA-GO content

PDA-GO content/wt%Tensile strength/MPa Elongation at break/%Young’s modulus/GPa 0 36±4 2.5±0.1 3.48±0.19 0.25 51±3 3.2±0.2 4.57±0.15 0.50 68±2 4.1±0.1 6.78±0.19 0.75 56±5 3.1±0.1 5.44±0.16 1.00 56±4 2.8±0.2 4.70±0.20

3.3 Conductivity of the conductive paper

Fig.3 Conductivity of conductive papers with different PDA-GO content

In this study, the conductive paper was prepared by mixing OTCNC and PDA-GO suspensions and casting the mixture. As a matrix, the nanoparticlesized OTCNC can make the PDA-GO easily to form a conductive network in the material, while the rigidity of OTCNC also imparted good mechanical properties to the paper. In this way, the conductivity of the paper reached as high as 10-5 S/cm, with a PDA-GO content of 1 wt%, while the Young’s modulus of the paper was～4.5 GPa. It is proposed that this conductive paper could be used in soft electrical devices or electrical signal-based sensors.

4 Conclusions

It has been reported that the conductivity of GO is 4.60×10-8 S/cm[24]. The prepared conductive paper benefited from the partial reduction of GO and the conductivity of PDA, and showed an increase in conductivity from about 10-9 S/cm to 10-5 S/cm with the increasing PDA-GO content (Fig.3). The catechol groups on PDA made the molecule conductive. Owing to the continuous network formed by PDA-GO, the conductive PDA molecules could further improve the conductivity of the paper. The low conductivity of GO could be improved by its reduction to RGO[15].Polymerization of dopamine also induced the reduction of the GO sheets, and thus increased the conductivity of the PDA-GO network. As seen in Fig.3, the conductivity of the paper increased dramatically from 2×10-6 S/cm to 1.3×10-5 S/cm when the PDAGO content increased from 0.75 wt% to 1 wt%, and increased slightly with further increase in the PDAGO content. This change indicated that a continuous network was formed when the PDA-GO content was～1 wt%. This concentration was low as compared to previous research[7-11]. Based on the observed changes in the conductivity of the paper when the PDA-GO content was more than 1 wt%, it was predicted that the conductivity of the paper would increase further with increasing PDA-GO content as more electrically conductive paths would form. This result indicated that the OTCNC matrix was better for the formation of continuous conductive networks in the conductive sheets, and for the production of conductive papers with good conductivity and mechanical properties.

Benefiting from the multiple hydrogen bonds formed between PDA-GO and OTCNC and among OTCNCs,the cast conductive paper showed good mechanical strength. However, as a result of the rigidity of OTCNC, the extensibility of the paper was low. Table 1 shows that with an increase in the content of PDA-GO,the Young’s modulus of the film first increased from 3.48 GPa to 6.78 GPa, and then decreased to 4.70 GPa.This was mainly because with an increase in the PDAGO content, the catechol group on the PDA-GO surface interacted strongly with the carboxyl and hydroxyl groups on the OTCNC surface to form hydrogen bonds.This hybrid network made the paper more flexible and rigid; however, a further increase in the PDA-GO content led to self-assembling of PDA on the PDA-GO surface, causing the PDA-GO to aggregate. Aggregation led to an uneven dispersion of PDA-GO in the OTCNC matrix. Meanwhile, the continuous network of PDAGO partially broke the OTCNC network and decreased the rigidity of the paper. It is worth noting that the interaction between OTCNC and PDA-GO was not specific. The OTCNC network could slide on the PDAGO surface under an external force applied to the paper,thus decreasing the rigidity and extensibility of the paper.

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Different amounts (50, 100, 150, and 200 mg) of PDA-GO were added to 1 wt% OTCNC suspension (20 mL)and stirred for 3 h. The mixtures were then poured into Teflon molds and heated for 12 h at 60℃ to obtain conductive papers with different GO mass ratios.

Authors are grateful to the National Natural Science Foundation of China (51373131), Fundamental Research Funds for the Central Universities(XDJK2016A017 and XDJK2016C033), Project of Basic Science and Advanced Technology Research,Chongqing Science and Technology Commission(cstc2016, jcyjA0796), and the Talent Project of Southwest University (SWU115034).

References

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Illustration of Cover Photographs

Vertical-assembly of cellulose nanocrystals for information hiding and reading: Saving the information in the optical materials, which can display various designated states in alternating conditions, is an important method of information security. Fluorophores and stimulation-responsive dyes are two typical materials in this method,whereas they usually suffer problems of photobleaching and aggregation-induced quenching. By contrast, the assembled materials with the periodicities equal to the wavelength of visual light own unique structural colors,which are free of the problems mentioned above. In this case, the structural color of cellulose nanocrystalassembled films were successfully controlled in ultraviolet region, which could hide the information of arrayed cellulose nanocrystals-based films under natural light. Periodical arrays of cellulose nanocrystals could even enhance scattering, which was similar to the photonic crystals. This feature made the hided information of the cellulose nanocrystal-assembled films readable as enhanced scattering blue light under an ultraviolet radiation. By an evaporation-induced method, cellulose nanocrystals (shown in the TEM image of front cover,the left block diagram) could arrange vertically to the solution plane, which removed the usual chirality and iridescence of assemble films based on rod-like cellulose nanocrystals. This ensured the specific designation of structural color as monochromatic light for the vertical-assembled cellulose nanocrystal films (the arrangement of cellulose nanocrystals can be seen in the AFM image of front cover, the right block diagram), which prevented the information from being misread due to iridescence.

提供对供应商监督评价信息的录入、查询功能；实现供应商评价功能，可对供应商从技术指标或参数、供货质量、完成进度（按期到货率）、供应商性质等方面进行综合评价，系统可提供量化的统计数据以供参考；实现供应商货源清单、供应商按期到货率、供应商供货质量分析、供应商供货进度跟踪。

Corresponding author: Jin Huang, professor.

School of Chemistry and Chemical Engineering, Joint International Research Laboratory of Biomass-Based Macromolecular Chemistry and Materials, Southwest University, Chongqing, 400715, China

莫言之所以能够在高密东北乡这块土地上任意泼墨挥洒，关键在于福克纳对他的启发。偶然的机会莫言阅读了福克纳的《喧嚣与躁动》，书中福克纳虚构了约克纳帕塔法县，莫言如梦初醒“他的约克纳帕塔法县是完全虚构的，我的高密东北乡则是实有其地。我也下决心要写我的故乡那块邮票大小的地方。这简直就像打开了一到记忆的闸门，童年的生活完全被激活了”。④灵感不断地从莫言的脑海中涌出，故乡的亲人、故乡的高粱地……故乡的一切被再现出来，记忆在他的笔下流淌。

E-mail: huangjin2015@swu.edu.cn; huangjin@iccas.ac.cn